Update style and layer metadata in Style#setState

[anandthakker]

Currently, diffStyles just ignores changes in metadata properties. In light of smart setStyle, this may betray users' expectations, because after calling Map#setStyle with style JSON where metadata properties have changed, Map#getStyle() will currently yield a serialized style with the old metadata properties.

[anandthakker]

Nope, I was wrong -- because of

mapbox-gl-js/js/style/style.js

Line 364 in d0543e6

this.stylesheet = nextState;

, we pick up the new stylesheet and its metadata just fine.